Semiconductor and other electronic devices are typically manufactured by creating multiple copies of a device together on a substrate and then singulating the devices. Substrates can be comprised of silicon, sapphire, ceramic, glass or glass-like material. Singulation is the act of separating multiple devices manufactured on a substrate from each other by creating a cut or break between the devices to form individual devices. Singulation can be performed in many ways. One, called scribing/breaking, is accomplished by first machining a cut or trench in the surface of the substrate or material attached to the surface of the substrate without cutting through the substrate. This is followed by mechanically breaking or cleaving the substrate to separate the die. This cut or trench can be formed by mechanical sawing, chemical etching, laser radiation or a combination. Another way to singulate substrates is called dicing, where the entire depth of the desired separation is performed by mechanical sawing, chemical etching or laser radiation or a combination, without mechanical cleaving.
Semiconductor devices are typically arranged in rows and columns on a substrate, with areas free from critical components between them. These free areas are called “streets” and typically form two sets of straight lines arranged perpendicularly to each other in order to yield rectangular devices once singulated. FIG. 1 shows a typical semiconductor wafer 10 comprised of compound devices, one of which is indicated 12 and streets, one of which is indicated 14. Also shown is top surface metal, one of which is indicated 16.
Semiconductor devices are becoming more complex as consumer's desires for more functions in smaller and smaller packages are encouraging manufacturers to put more circuitry into less volume. Three trends emerge from this increase in packaging density. First, substrates are getting thinner, making them more susceptible to damage from cracks or chips. Second, circuit density is getting greater and as substrate space becomes more valuable, there is a desire to minimize the area devoted to streets. Finally, electronic devices are increasingly packaged as compound devices wherein multiple substrates are assembled prior to singulation in order to take advantage of the parallelism inherent in wafer scale integration and package more capability into a smaller volume or to protect underlying components.
Compound semiconductor devices can be made of multiple substrates, possibly containing multiple semiconductor devices, assembled into a single functional device. These compound devices are often assembled prior to device singulation and can present problems for typical singulation methods. FIG. 2 shows an exemplary compound device, in this case a complementary metal-oxide semiconductor (CMOS) image sensor, from a side view, showing a silicon wafer 20 containing active circuit elements (not shown), solder bumps, one of which is indicated 22, and top surface metal, one of which is indicated 24. The wafer 20 is separated from the glass cover plate 26 by shims, one of which is indicated 28. One of the streets is indicated 30, as the region between the adjacent dotted lines. In this device, the image sensor is on the surface of the silicon wafer 20 that faces the glass cover plate 26. The transparent cover plate protects the sensor while allowing light to reach the sensor.
One prior art process for singulating semiconductor devices is to cut the wafers with a mechanical saw, for example the DISCO DAD3350 (DISCO Corporation, Tokyo, Japan). One of the disadvantages of mechanical saws is they can cause chipping and cracking along the kerf or cut they create. These chips and cracks weaken the substrate and can cause problems with the eventual semiconductor device. This weakening of substrates due to chipping and cracking becomes worse as substrates become thinner. Mechanical saws also have a minimum kerf size based on the minimum width of the saw. This limits the manufacturer's ability to reduce street size and improve the usable area of the substrate. One possible solution to the cracking problem is to slow down the rate at which the saw moves through the substrate. Careful adjustment of cutting pressure and speed of the saw through the material is required to avoid creating cracks, both of which cause decreases in throughput, since less pressure means more passes with a mechanical saw and slower speed required more time per pass. While this can reduce the cracking and chipping problems, it can slow down the singulation process unacceptably.
An alternative process for singulating semiconductor devices is to use a chemical or plasma etching to form the cuts. An example of this is shown in U.S. Pat. No. 6,573,156, assigned to OMM, (OMM, Inc., San Diego, Calif.). In this process a trench is etched on one side of a wafer, a temporary holding material is applied to that side and the opposite side is then etched. The temporary holding material is them removed to permit the individual semiconductor devices to be separated. This method has the disadvantage of requiring several additional steps and additional equipment to be added to the manufacturing process, thereby increasing manufacturing cost and time.
Another common method of making the cuts is with a laser beam. Laser beams are also capable of making non-straight cuts in cases where device outlines or streets are not straight or perpendicular. Singulation can be accomplished by using a laser beam to machine completely through the substrate forming a through-cut, thereby separating the devices (dicing), or by making a scribe or partial cut into the substrate, which is subsequently mechanically cleaved or fractured from the bottom of the scribe to the opposite surface of the substrate (scribing/breaking). Singulating semiconductor devices with a laser beam can reduce the substrate area devoted to streets, since the laser beam can machine a kerf smaller than the smallest saw blade. The laser beam is capable of either cutting a kerf or creating a scribe in the materials that typically make up compound semiconductor devices such as silicon wafers and glass cover plates. These kerfs or scribes can be smaller than the 100 microns typically made by mechanical saws. FIG. 3 shows a glass substrate after being scribed with nanosecond laser beam pulses. In this case a pulsed, frequency doubled Nd:YAG laser is used to direct laser pulses at a glass substrate to form scribes. The laser operates at 532 nm, emitting pulses with a temporal pulse width of about 30 nanoseconds (ns), energy of about 1 millijoule (mJ) per pulse, a Gaussian spot size of about 15 microns and a pulse repetition rate of about 10 KHz. FIG. 3 shows a photograph of a glass substrate 36 with laser machined scribes, one of which is indicated 38. Note that the scribes show strong chipping 39 along the edges of the scribe. Chipping is undesirable because it can weaken the glass and cause it to fail following packaging. Chipping can occur with both scribing and dicing.
Another problem with scribing or cutting glass with nanosecond-scale laser pulses is the creation of cracks. FIG. 4a shows a cross-sectional view of a laser scribe in a glass substrate 50 scribed with the same laser parameters as the scribes in FIGS. 3 and 4. This image clearly shows the heat affected zone (HAZ) 52 surrounding the scribe 54. In addition, a void 56 and stress fractures 58 are shown near the bottom of the scribe 54. These defects, likely caused by thermal effects from the nanoseconds long laser pulses, will cause cracks when the glass substrate is cleaved following scribing as shown in FIG. 4b. FIG. 4b shows a side view of the substrate 50 from FIG. 4a following cleaving, with the scribe 54 on the right and representative cracks 60 shown propagating from the area of the scribe 54. These cracks 60 are a result of cleaving the substrate 50 in the presence of voids and stress fractures as exemplified in FIG. 4a 56, 58. Following packaging, cracks such as these can potentially propagate, causing device failure. Mechanical saws can also create this type of crack.
A process designed to overcome this chipping and cracking is to use a chemical or plasma enchant to “melt” the edges to attempt to remove the effects of the mechanical saw. An example of this is described in US patent application 2006/0249480 Laser Machining Using an Active Assist Gas. In this application, a process of using a halogen assist gas to help reduce chipping and cracking from resulting from laser machining substrates is shown. This application indicates an increase in die strength by reducing chipping and cracking, but at the cost of the additional equipment required to safely deliver and exhaust caustic gas to and from the work area.
Compound devices present additional problems when singulated by existing methods. In the case of mechanical saws, the saw would have to cut through the street 30 between the dotted lines in FIG. 2, through both the silicon wafer 20 and the glass cover plate 26. A particular problem with compound devices is the quality of the kerf. Sawing parameters which work for one material may not be optimal for other materials. Since compound devices, particularly image sensors are made of substrates of dissimilar materials, one set of sawing parameters, such as a particular speed of cut, saw rotation speed or pressure or one set of laser parameters such as pulse energy or repetition rate may be good for one substrate material but not the other. Glass, generally being more brittle than silicon wafers, has a tendency to chip or crack along the kerf when it is sawn or laser processed. In order to prevent chipping and cracking, the speed at which the diamond saw travels while sawing the glass substrate has to be much slower than when sawing silicon alone. This can also be true when laser processing. This has deleterious effects on system throughput. In addition, mechanical saws have a minimum width due to mechanical factors thereby establishing a minimum width for the streets that must contain the kerfs formed when sawn by mechanical saws. Typical minimum width for mechanical saw kerfs are about 100 micron. Since streets represent wasted space on substrates that could otherwise be used for active device elements, there is a desire to minimize the space devoted to streets and increase the space available for active device elements.
In accordance with the information presented above, there is therefore a continuing need for a method of singulating compound semiconductor devices that prevents chipping or cracking damage to the device, avoids adding extra chemicals, equipments and manufacturing steps to the singulation process and maintains system throughput when compared to prior art methods.